1. Field of the Invention
This invention relates to rigid framing systems, and more particularly to framing systems achieving rigid frame action by use of non-bonded connections.
2. Description of the Prior Art
Frame rigidity is desirable because it facilitates economy of materials, simplicity of configuration and physical stability in the support of horizontal and vertical service loadings.
A principal requirement for rigid frame action is rigid connections that are capable of immediate moment transfer upon application of service loads. The majority of such connections are of the bonded type, such as welding of metals, monolithic casting of concretes, and the use of adhesives, as with wood. Such connections are non-adjustable and require close quality control to insure integration of connected members.
Non-bonded connections and fastening assemblies presently used in forming frames vary in their rigidities, the degree of rigidity being inversely proportional to the amount of non-moment transferring rotation in the joints. Most non-bonded joints require widely varying amounts of rotation before they function in full moment transfer and the connections thereof are non-adjustable.
Non-bonded connections usually employ mechanical fasteners, such as unfinished bolts, hot or cold driven rivets, high strength bolts, nails, screws, and other special connectors. The imperfect fit between the unfinished bolts or rivets and their respective holes results in unreliable moment transferring connections. The same applies to all connections between wood members, where the fit between connectors or fastening assemblies and the wood are subject to degradation, mainly due to the shrinkage and deformation of the wood. Though non-bonded connections simplify assembly and disassembly of frames, they lack the capability of adhesives for rigidly joining unlike materials and shapes.
Generally, the degree of rigidity and the functioning of prior art connections using mechanical fasteners is sensitive to the proper fit of connectors, and also to the proper alignment of members and a multiplicity of connector elements. Improper alignment may cause prying action in the joint which tends to reduce joint rigidity, especially in the so-called friction-type bolt connections using high strength bolts. Prior art non-bonded connections are usually quite cumbersome and bulky, and require a considerable degree of precision in fabrication and assembly of complex elements and frame members.
Close tolerance in fabrication is more difficult to attain with precast concrete than with wood or metals, and the degree of connection and frame rigidity attained in non-bonded concrete frames leaves much to be desired. For example, Canadian Letters Pat. No. 467,791, in the name of Albert Henderson, teaches a modular system of standardized structural members intended to insure more exact positioning or space relation of the elements of his connections. Brackets are provided to seat the beams, and for shear support, and the need for high tensioning of the bolts of the friction-type connection is emphasized. However, the bolts pass through steel pipe sleeves set in the concrete members and welded to their reinforcing bars, the sleeves being specified as having a 2 inch diameter. The loose fit inevitable from such tolerance will cause a loss of joint rigidity and is typical of connections for precast concrete. U.S. Pat. No. 3,495,371, in the name of N. B. Mitchell, Jr., discloses another approach to precast concrete connections wherein the beam members are clamped into steel crotches or troughs fabricated on the column tops. Here, too, joint rigidity is adversely affected by the loose fit of the tension bolts in the sleeves and holes, and by any unevenness of the faces of the concrete beams where they adjoin the faces of the steel trough. Conventional bolted connections are specified for beams at right angles to the moment transfer connections, so that these frame structures could be rigid in only one direction.
The structural uses and advantages of camber and prestressing are well understood. These features are usually incorporated in the structural members during fabrication and cannot thereafter be adjusted. U.S. Pat. No. 2,626,688, in the name of A. S. T. Lagaard, however, teaches the use of co-acting spacers in the top chord of joists for limited field adjustment of chamber prior to application of service loads. The use of prefabricated tensioning and camber generally has no bearing on the rigidity of the connections when the members are assembled into frame structures.
U.S. Pat. No. 3,070,845, in the name of D. B. Cheskin, discloses a pre-stressed connection for joining a series of beams into a continuous beam while imparting tension and camber into the component beams. The continuous beam is supported on columns, but the specified connections do not engage the columns, or other beams at right angles to the continuous beam, in the moment transfer condition necessary for rigid frame action. Addtionally, a second bolt or set of bolts is required at each connection to anchor each beam to its supporting columns, but once secured, such anchor bolt resists the relative rotation of the beam members in a manner that limits the stated objectives of tensioning and cambering. Cheskin discloses one alternative design of connection in FIG. 8 (to provide for tensioning of an end beam from both ends) which could be capable of moment transfer between the beam and the column, but this design can be used only on the outermost columns of a series, providing a limited contribution to the rigid frame action of the structure along only one of the two axes of the building. It should be emphasized that Cheskin does not mention frame rigidity nor multispan rigid frames in two- and three-dimensional configurations. In addition, the configuration of the specificed connections limits their practical usefulness to one story structures.